In the float process forming flat glass, molten glass is drawn from a melting furnace and passed to a forming chamber (or "float bath") where the molten glass is deposited onto an elongated pool of molten metal such as tin or copper or alloys thereof. There, a ribbon of glass is stretched to the desired thickness as it progresses along the elongated pool of molten metal and is then withdrawn from the forming chamber as a continuous ribbon at the exit end of the forming chamber. Because of the fluid support provided by the molten metal to the glass, glass of superior optical quality can be produced by the float process.
Unfortunately, a float glass forming chamber is not free from distortion producing effects. One such effect is the "drip" problem which is caused by dripping of molten droplets of metal or compounds thereof from the roof of the forming chamber onto the glass ribbon. Although the atmosphere within the float forming chamber is usually positively pressurized with an inert or reducing gas atmosphere, sulfur and oxygen are introduced into the chamber from the glass ribbon and from other sources, and these combine with the metal of the molten metal bath to form sulfides and oxides (e.g., tin sulfide and tin oxide) which volatilize and condense on relatively cool portions of the interior surface of the float chamber. The condensate accumulates on the structural members of the bath interior, and under certain temperature and chemical conditions will be reduced to elemental metal (e.g., tin), which eventually falls as droplets onto the glass ribbon. The impact of the metallic droplets on the soft glass ribbon produces indentations which appear as optical distortions in the final glass product. This defect is known variously as "tin drip," "crater drip," "top drip," or "tin speck."
It has now been found that the drippage problem is aggravated by the roof configuration conventionally employed in float forming chambers. The roof design commonly in use comprises a complex grid of relatively small ceramic pieces interlocked with one another and suspended from above by a large number of metallic rods. The design includes a large number of vertically extending electrical heating units supported within openings in the grid. Other openings in the grid are filled with blind plugs. The result is an interior roof surface which is non-planar and has a relatively large surface area and a large number of joints and vertically extending cracks and surfaces. Such a complex roof structure encourages condensation and dripping of volatilization products. The large number of crevices permits ingress of cooler exterior atmosphere which promotes condensation. The non-planar surfaces tend to increase running and coalescing of condensation products. More recent designs of float bath roofs have simplified the support grid design so as to extend across the float chamber in only the transverse direction for the sake of simplified construction. However, the revised design still possesses the drawbacks of a large number of joints and non-planar interior surfaces.